This project is concerned with two major areas of investigation focussed upon functions of the brain stem: (1) Chemosensory mechanisms involved in the respiratory regulation of blood CO2 and H ion concentrations; (2) Chemosensory mechanisms involved in the control of vomiting. Experiments are performed on chronic unanesthetized cats and on acutely decerebrated cats. Our objectives are to quantify the chemosensory reflex behavior from a control-systems point of view, to functionally isolate the chemosensors in order to define their input-output characteristics, and to identify morphologically the participating receptor elements.